It's hard to imagine such a scenario as I think the main reasons The Supremes were signed to Motown was because Berry found Diana's voice has the potential to be very lucrative and as groups were a big deal at the time, she was signed as part of a group. If groups weren't a big deal, he would have signed her solo, and she would have happily ditched the other girls. Only thing preventing that would have been her parents not going along with the plan.

If Berry and Motown had lost patience by 1963 of Diana and the group becoming a hit making act, the whole group would have been dropped from the label. Flo would not have been designated lead singer with Diana in the background. Only way Flo would have been designated lead singer would have been if Diana dropped out of the group, and Berry and Motown decided, for whatever reason, to try Flo, Mary and the new member as The Supremes. But they would have likely been given one or two chances to hit and if not they would have been dropped.

And only scenarios I could imagine someone as drive as Diana leaving the group in 1963 would be: 1--got pregnant and being family oriented as she is decided she was fulfilled raising a child and having more; 2--her parents gave her an ultimatum and demanded she quit singing and go to college and find a profession and being family oriented she complied; 3--she didn't let her foot off the accelerator and ran over Gladys Horton and the blind child and ended up going to prison to serve a sentence. In that light, I doubt Motown would have continued The Supremes as they would have been known soley as the group with the crazy girl who was so jealous of The Marvelettes and their success she murdered their leader. In that case maybe Flo and Mary would have stayed on at Motown with surviving members of the Marvelettes under a new group name.